A silver halide emulsion used in a silver halide photographic material is usually chemically sensitized with various chemical substances in order to obtain a prescribed sensitivity and gradation. Typical methods for doing so include various sensitizing methods such as sulfur sensitization, selenium sensitization, tellurium sensitization, noble metal sensitization such as gold sensitization, reduction sensitization, and combinations thereof.
In recent years, there has bean a strong demand for high sensitivity, excellent graininess and high sharpness in a silver halide photographic material as well as for rapid processing in which development processing is expedited. Various improvements in the above sensitizing methods have been made.
Of the above sensitizing methods, the selenium sensitization and tellurium sensitization are disclosed in U.S. Pat. Nos. 1,574,944, 1,602,592, 1,623,499, 3,297,446, 3,297,447, 3,320,069, 3,408,196, 3,408,197, 3,442,653, 3,420,670, 3,591,385, 3,772,031, 3,531,289, and 3,655,394, French Patents 2,093,038 and 2,093,209, JP-B-52-34491 (the term "JP-B" as used herein means an examined Japanese patent publication), JP-B-52 34492, JP-B-53-295, and JP-B-57-22090, JP A-59-180536 (the term "JP-A" as used herein means an unexamined published Japanese patent application), JP-A-59-185330, JP A-59-181337, JP-A-59-187338, JP-A-59-192241, JP-A-60-150046, JP A-60-151637, and JP-A-61-246738, British Patents 255,846, 861,984, 235,211, 1,121,496, 1,295,462, and 1,396,696, Canadian Patent 800,958, and Journal Photographic Science, Vol. 31, pp. 158 to 169 (1983), written by H. E. Spencer et al.
However, while the selenium sensitization has a greater sensitizing effect than the sulfur sensitization usually applied in the art, it has a marked tendency to cause too much fog and soften a gradation. Many of the above-cited patents improve the above defects, but the results obtained are still insufficient. In particular, a basic improvement to control the generation of fog has been intensively desired.
In particular, the combination of gold sensitization with sulfur sensitization or selenium sensitization can provide a marked increase in sensitivity, but at the same time there is an increase in fog. The gold-selenium sensitization particularly causes the fog to increase compared to gold-sulfur sensitization. Accordingly, there has been an intensive search for selenium sensitizers in which sensitivity change during storage is controlled nd the generation of fog is suppressed.